089
FXUS65 KRIW 041807
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1107 AM MST Thu Dec 4 2025

.KEY MESSAGES...

- Breezy winds and seasonable temperatures are forecast through
  the daylight hours today.

- A multi-day winter weather system, bringing the first
  significant snowfall to much of western Wyoming, begins later
  this afternoon and continues through Saturday. The heaviest
  period of snowfall is expected to occur Friday evening through
  Saturday morning.

- Widespread breezy to gusty winds are expected Friday afternoon
  through the overnight hours. Gusts of 25 to 35 mph will be
  common across the Wind Corridor with wind prone locations
  seeing gusts around 50 mph.

- Outside of light snow chances (30-80%) for western Wyoming,
  drier and warmer conditions is looking favorable late Saturday
  through early next week.

&&

.DISCUSSION...
Issued at 343 AM MST Thu Dec 4 2025

10Z water vapor imagery shows strong north-northwest flow aloft with
ridging over the Pacific and a broad trough to the east. An embedded
weak disturbance is entering the northwest portions of the state.
This will provide weak support for orographically-induced light snow
chances (20%-40%) across higher elevations of Yellowstone National
Park and the Teton Mountains through just after sunrise. Some breezy
20 to 30 mph winds will also be present from Rock Springs to Casper
as the weak disturbance traverses the state through the morning.
Seasonal temperatures are forecast today ahead of the incoming
weather system.

By the early afternoon, the first "wave" of this multi-day snow
across the west and widespread gusty winds will approach. This first
wave, currently (10Z) moving onshore over the Pacific Northwest,
will mostly be a wind maker across the Absarokas (including the Cody
area) , upper Wind River Basin, and across the Wind Corridor. This
wind will be a result of a tightening pressure gradient ahead of the
second "wave" and associated cold front later tonight. Gusty winds
of 30 to 35 mph will be common across the aforementioned locations,
with isolated gusts of 50 mph possible. Snow with this first wave
will be confined to western Wyoming. Snow amounts this afternoon
(from 11AM to 5PM) are forecast to be up to 0.5 inch across Jackson
and Star Valleys and much of Yellowstone. The Tetons and high
elevations of Yellowstone could see around 2 inches during this time
frame.

The second wave begins around sunset tonight as upper level
divergence is enhanced over western Wyoming, thanks to being in the
left exit region of a 125kt-150kt upper level jet. With abundant
moisture filtering into the region and enhanced lift, due to
orographics and upper level divergence, moderate to heavy snow is
expected from sunset tonight through early Friday morning for the
western mountains. During this time frame, snow amounts across the
Teton, Gros Ventres, Salt River, and Wyoming Mountain Ranges and
high elevations of Yellowstone range from 6 to 12 inches. As for the
western valleys and low elevations of Yellowstone, snow amounts
overnight tonight generally range from 1 to 3 inches.

A lull in moderate to heavy snow is currently forecast between
sunrise and midday Friday as the upper level jet shifts east over
Wyoming. However, light snow will likely (90% chance) be falling
during this time. Once the 125kt-150kt jet is overhead, the third
wave begins early Friday afternoon and continues through Saturday
morning. During this time, moderate to heavy snow is forecast across
most of western Wyoming. Snow totals across the Teton, Gros Ventres,
Salt River, and Wyoming Mountain Ranges range from 1 to 2 feet with
the highest elevations seeing 3 feet. Thus, the Winter Storm Watch
has been upgraded to a Winter Storm Warning beginning 5PM tonight
and continuing through 2PM Saturday. Travel over Teton, Togwotee,
and Salt River Passes could become very difficult, especially during
both "waves".

Winter Weather Advisories have also been issued for the Upper Green
River Basin, the western Wind River Mountains, the Absarokas,
Yellowstone, and Jackson and Star Valleys. Total snow amounts of at
least 7 inches (80-90% chance) are forecast for Jackson and Star
Valleys, Yellowstone, and the Upper Green River Basin. For these
locations, the probabilities of at least 8 inches ranges from 40% to
80% over a 40+ hour period so opted only for Winter Weather
Advisories at this time. Higher elevations of Yellowstone
(Pitchstone Plateau) could see upwards of 2 feet, however, because
these areas are largely untraveled, kept with the Advisory. Similar
thinking swayed the thinking for the western Wind River MOuntains,
the Absarokas, and the western Bighorns, with snow amounts between 8
and 12 inches. Locally higher amounts are possible (70%) across the
highest elevations of these mountain ranges. It is important to note
that gusty winds look to accompany the moderate to heavy snow,
leading to reduced visibilities.

With the strong upper level jet moving overhead Friday afternoon,
winds begin to ramp up across the Wind Corridor and the Absarokas
with frequent 25 mph to 35 mph gusts. This will likely (80-90%
chance) translate to frequent gusts of at least 35 to 40 mph.
Wind prone locations, such as South Pass and Muddy Gap, could
potentially (50-80% chance) see gusts around 50 mph. Gusty 30 to
40 mph drainage winds across the upper Wind River Basin look to
develop (90% chance) late Friday into Saturday as well. As the
jet moves overhead, 700mb winds of 50kts to 60kts are forecast
to develop over southern Wyoming, the Absarokas and the Upper
Wind River Basin late Friday evening into the overnight hours.
There is a 50% chance for wind gusts 60+ mph to occur in these
areas as a result of the jet max moving overhead and mixing
these stronger winds to the surface. Similar conditions look to
develop through the rest of the Wind River Basin and northern
Johnson County between 09Z and 12Z Saturday. These potentially
high winds could last through Saturday morning, before gradually
decreasing after 21Z Saturday.

The upper-level jet begins to weaken and shift south during the day
on Saturday, bringing an end to the impacts of the winter system.
Ensemble guidance is in consensus that the northwest flow will
persist Sunday into the early parts of next week, allowing for light
snow to chances (30-80%) continue across the west. Elsewhere, dry
conditions and a warm up to above normal temperatures is also
looking more favorable.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1106 AM MST Thu Dec 4 2025

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals.

Light snow and MVFR conditions linger at KJAC late this morning. An
improvement to VFR is expected early this afternoon though will have
to watch trends closely for a possible extension of light snow
impacts. Remaining terminals will be VFR through the afternoon with
a gusty west wind at KRKS. More persistent snow will push into
western Wyoming by sunset. This will bring deteriorating conditions
at KJAC around this time, with IFR conditions expected by 01Z. KJAC
is then expected to persist IFR through most of Friday morning.
Confidence is lower on snow making it farther east, with KPNA having
the best chance of MVFR conditions overnight. KRKS will continue to
see a west wind through the period with gusts generally over 20
knots. Frequent mountain obscurations are expected through the
period.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals.

Snow will have a more difficult time moving east of the Divide. The
primary impact from this system will be strong wind at KCOD. Gusts
will peak over 40 knots through this evening, dropping only slightly
overnight. KCPR will see a southwest wind through the daylight hours
today gusting to around 30 knots. Otherwise, terminals will remain
VFR through most of tonight. Cloud decks will lower around 12Z
Friday, with MVFR ceilings expected at KCOD, KCPR, and KWRL through
the end of the period. KCPR will have the best chance of light snow
late in the period and have included a PROB30 group to account for
this.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 PM this afternoon to 2 PM MST
Saturday for WYZ001-002-008-013-014-023-025.

Winter Storm Warning from 5 PM this afternoon to 2 PM MST
Saturday for WYZ012-024.

&&

$$

DISCUSSION...Gerhardt
AVIATION...Myers